Subscription Service Set |
This topic contains the following sections:
The Subscription Service Set is used by the Client to create and maintain Subscriptions. Subscriptions are entities that periodically publish NotificationMessages for the MonitoredItem assigned to them. The NotificationMessage contains a common header followed by a series of Notifications. The format of Notifications is specific to the type of item being monitored (i.e. Variables, Attributes, and EventNotifiers).
Subscription Service SetOnce created, the existence of a Subscription is independent of the Client’s Session with the Server. This allows one Client to create a Subscription, and a second, possibly a redundant Client, to receive NotificationMessages from it.
To protect against non-use by Clients, Subscriptions have a configured lifetime that Clients periodically renew. If any Client fails to renew the lifetime, the lifetime expires and the Subscription is closed by the Server. When a Subscription is closed, all MonitoredItems assigned to the Subscription are deleted.
Subscriptions include features that support detection and recovery of lost Messages. Each NotificationMessage contains a sequence number that allows Clients to detect missed Messages. When there are no Notifications to send within the keep-alive time interval, the Server sends a keep-alive Message that contains the sequence number of the next NotificationMessage sent. If a Client fails to receive a Message after the keep-alive interval has expired, or if it determines that it has missed a Message, it can request the Server to resend one or more Messages.
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